The present invention relates to a method and an apparatus for moving a medium, e.g. paper, through a printer or other device. More particularly, the present invention relates to providing a motion profile for aggressive/accurate medium indexing through a printer or other device requiring such medium indexing.
As demand for thermal ink jet printers (xe2x80x9cTIJ printersxe2x80x9d), for example, increases due to their flexibility and pricing, so too does the expectations of end users. A medium, e.g., paper, is passed through the printer at a constant rate, where a constant ramp in acceleration is used as a motion profile in order to stress indexing speed over smoothness of motion. However, this constant acceleration fails to take into consideration the availability of higher motor torque available at lower motor speeds and demands as much torque at high speeds, which often places demands on the motor torque and power that cannot be met. Other traditional motion profiles favor smoothness of motion, i.e., less acceleration at both low and high speeds, but these motion profiles undesirably result in a longer indexing time, which lowers the operational speed of a printer device.
As TIJ carriage printers begin to use higher carriage speeds and larger heads, it becomes imperative to use a more aggressive method to advance or index the medium between lateral print head passes across the medium. Less time is available during traditional carriage acceleration and deceleration, and the medium must be moved farther. Traditionally, the medium is advanced by a stepper motor driving a gear train to drive wheels or rollers engaged with the back side of paper. Stepper motors have proven to be ideal choices for low cost indexing mechanisms.
However, for an medium indexing system contemplated by this invention, it was required to advance paper, for example, a distance of 1.57 inches in approximately 0.15 seconds. This amounted to an average speed of 10 inches per second with a much higher peak speed near the middle of the indexing intervals. Although stepper motors may be used, the present invention preferably uses a servo motor. Stepper motors, especially those typically used in less expensive TIJ printers, may not be capable of reaching the desired, intermittent indexing speeds.
Initial testing showed that it was necessary to run the servo motor at maximum speeds in order to attain favorable torque stiffness at the drive wheel to generate accurate indexing. Gearing is selected to xe2x80x9cwindxe2x80x9d the motor out when at peak speed. Thus, if a motion profile, i.e., velocity-versus-time or acceleration-versus-time profile, could minimize the peak motor speed, it would optimize the indexing by allowing a more favorable gearing.
Both servo motors and stepper motors demonstrate greatly diminished torque at higher rotation speeds, assuming a constant drive level. When either type of motor is driven at high speeds, the servo controller (using an encoder feedback) demanded very large currents from its power supply. This creates an undesirable and potentially severe power supply cost problem.
Traditionally, a constant acceleration ramp for the motion profile is used, especially when desiring to stress indexing speed over smoothness of motion. However, this profile fails to take into consideration the relatively higher availability of torque at low motor speeds and the lack thereof at high speeds. This causes a problem with the motor""s power supply.
The present invention overcomes the above and other problems by providing a method of and an apparatus for driving a medium indexing motor in a certain manner in order to utilize relatively greater available motor torque at low motor speeds during both acceleration and deceleration, and to use less torque at high motor speeds. Such a driving profile has the advantage of reducing the peak motor speed necessary to advance the medium the required distance in the minimum amount of time.
It has also been found that the motion profile is optimized by the addition of a discontinuity in the velocity of the medium driven by the motor. The addition of the discontinuity to velocity prevents the medium from overshooting its targeted stop location.
A method according to the present invention includes driving a motor of a printer or other medium indexing device at such speeds as to make optimum use of the torque generated at relatively lower motor speeds and make less use of torque at relatively higher motor speeds. Moreover, such a method may further include adding a discontinuity to the velocity profile to prevent the substrate being fed through the printer from overshooting its desired stop position.